`v2.0.0-alpha.13` release preparation

[alejandro-vaz]

so we're releasing alpha.13 in a few days

big parts of the refactor have already been made, what's left for future alphas is:

• modularize iterators: they're very entrenched and depend on each other so it is chaos modularize associated iterators #479
• simplify the length model simplify the TaggedLen / set_len / capacity ... model #451
• possibly try the slice experiment experiment: change RawSmallVec heap to NonNull<[MaybeUninit<T>]> #441
• move documentation elsewhere add full feature list to ./README.md #491
• add support for allocators Support parametric allocators #55

we also need to figure out what code is necessary in the codebase, what can be simplified/refactored/removed, and shrink it. currently lib.rs still has 2.5k lines of magic

release checks

1. produce the automatic changelog, which is going to be massive
2. verify documentation builds, and if it doesn't, fix it
3. create tag + release

note that the release is scheduled for September 4th

must get merged

1. Add rayon support #548
2. refactor: split Eq/Ord and From/TryFrom impls out of lib.rs #545
3. feat: added borsh deserialize and tests #530
4. refactor: modularize CollectionAllocErr into errors.rs #514